1. Field of the Invention
The present invention relates to nickel-based amorphous alloy compositions, and more particularly to nickel-based amorphous alloy compositions, each of which forms an amorphous phase having a supercooled liquid region of 20 K or larger when cooled from a liquid phase to a temperature below its glass transition temperature at a cooling rate of 10.sup.6 K/s or less.
2. Description of the Related Art
Most metal alloys form a crystalline phase having a regular atomic arrangement upon being solidified from a liquid phase. However, some alloys can maintain their irregular atomic structure of the liquid phase in a solid phase when the cooling rate applied to the solidification is high enough to limit nucleation and growth of the crystalline phase. These alloys are commonly called as amorphous alloys or metallic glasses.
Since the first report of amorphous phases in Au--Si system in 1960, many types of amorphous alloys have been invented and used in practice. Most, however, of these amorphous alloys require very high cooling rates to prevent the crystalline phase formation in the course of cooling from the liquid phase because the nucleation and growth of the crystalline phase progress rapidly in the supercooled liquid phase. Accordingly, most amorphous alloys could be produced only in the form of a thin ribbon having a thickness of about 80 .mu.m or less, a fine wire having a diameter of about 150 .mu.m or less, or a fine powder having a diameter of a few hundred .mu.m or less by using rapid quenching techniques with the cooling rate in the range of 10.sup.4 to 10.sup.6 K/s. That is to say, practical applications of the amorphous alloys prepared by the rapid quenching techniques have been limited by the form and dimension thereof. Therefore, there has been a desire to develop alloys that require a lower critical cooling rate for avoiding the crystalline phase formation in the course of cooling from the liquid phase, that is, have a superior amorphous phase-forming ability so as to use the alloys in practice as common metal material.
If alloys have the superior amorphous phase-forming ability, it is possible to produce amorphous alloys in a bulk state by general casting methods. For example, in order to produce bulk amorphous alloys having a thickness of at least 1 mm, crystallization must be avoided even under the condition of a low cooling rate of 10.sup.3 K/s or less. For producing the bulk amorphous alloys, it is also important from an industrial point of view that the alloys have a large supercooled region in addition to the low cooling rate required for amorphous phase formation because viscous flow in the supercooled region makes it possible to mold the bulk amorphous alloys into industrial parts having specific shapes.
U.S. Pat. No. 5,288,344 and 5,735,975 disclose zirconium-based bulk amorphous alloys having the superior amorphous phase-forming ability, in which critical cooling rates required for amorphous phase formation are only a few K/s. Also, these zirconium-based bulk amorphous alloys are reported to have a large supercooled region, so that they are molded into and applied practically to structural materials. In fact, Zr--Ti--Cu--Ni--Be and Zr--Ti--Al--Ni--Cu alloys described in the specifications of the above patents are now used in practice as bulk amorphous products.
Considering, however, that zirconium is limitative in resources, has very high reactivity, includes impurities, and is very expensive, there has been a desire to develop bulk amorphous alloys containing a common metal, such as nickel, as a main constituent element which is more stable thermodynamically and more useful in industrial and economical standpoints.
Experimental results obtained from nickel-based amorphous ribbon show that nickel-based amorphous alloys have excellent corrosion resistances and strengths, which means that they can be applied to useful structural materials if only to be produced in the bulk state. A study reported in Materials Transactions, JIM, Vol. 40. No. 10, pp. 1130-1136 discloses that nickel-based bulk amorphous alloys having a maximum diameter of 1 mm can be fabricated in a Ni--Nb--Cr--Mo--P--B system by using a copper mold casting method, and these bulk amorphous alloys have comparatively large supercooled regions.
Nevertheless, for wider industrial applications of the nickel-based amorphous alloys, there is still a desire to develop new nickel-based bulk amorphous alloys that can be obtained in various alloy systems other than in the Ni--Nb--Cr--Mo--P--B system through proper alloy designs.